Highly integrated central processing unit (CPU) components such as the microprocessors available from Digital Equipment Corporation of Maynard, Mass. and Intel Corporation of Santa Clara, Calif. enjoy increasing popularity due to their high available processing speeds, low cost, and small size. However, because of the high integration of heat generating components, personal computer (PCs) systems and workstations developed around the use of such integrated circuits (ICs) usually require the use of cooling subsystems that typically include at least one fan. The fan provides forced air cooling of the IC components in order to prevent an over-temperature situation whereby damage might otherwise occur.
One manner of designing a cooling sub-system for a personal computer is to have the cooling fan simply run all of the time that the computer system is operating. However, many users find the noise generated by the fan to be objectionable. Knowing this, together with the fact that it is often the case that the CPU component simply sits in an idle or low utilization state during prolonged periods, system designers presently include control circuits that permit the fan to be slowed or even powered down at appropriate times. A typical design approach makes use of a temperature sensitive component, such as a thermistor, to provide an electrical voltage or current proportional to the present temperature of the CPU. A fan control circuit is then arranged to be responsive to overheating conditions, such as when the measured CPU temperature exceeds a predetermined temperature, and cycles the cooling fan to an on state. When the CPU returns to a safe temperature, the control circuit then shuts the fan off.